1. Field of the Invention
The present invention relates to a light deflector, an optical scanner, and an image forming apparatus.
2. Description of the Related Art
In optical scanners used in conventional electrophotographic image forming apparatuses, polygon mirrors or galvanometer mirrors are used as light deflectors for light beam scanning. In order to achieve higher image resolution and perform printing at higher speed, these mirrors should be rotated at higher speed. However, there is a limit to an increase in the rotational speed of the mirror (scanning speed) for reasons such as bearing durability, heat generation due to windage, and noise.
On the other hand, recently, studies have been made of a light deflector using silicon micromachining. For example, Japanese Patent Nos. 2924200 and 3011144 disclose techniques for forming a vibrating mirror and torsion beams, serving as a shaft to support the vibrating mirror, as a unitary structure using a Si substrate.
According to these techniques, it is possible to miniaturize a mirror surface, and resonance is employed to cause the mirror to vibrate back and forth or in a reciprocating manner. Therefore, these techniques have the advantage of enabling high-speed operation with low noise and low power consumption. Further, since the mass of a movable part is reduced, it is also possible to keep banding due to vibration at low level.
Japanese Patent Nos. 3445691 and 3543473 disclose employment of a vibrating mirror in place of a polygon mirror.
However, in order to configure an optical scanner supporting a “tandem system” according to which a color image is formed by superposing images of different colors recorded on corresponding photosensitive body drums using vibrating mirrors, a vibrating mirror is required for each image forming station.
Further, each vibrating mirror is driven independently. Therefore, a difference in resonant frequency results in non-uniformity of the scan line pitch, so that the degree of misregistration of scan lines gradually increases between the start and the end of writing in the sub scanning direction. Further, if the vibrating mirrors are different in vibration center, the magnification in the area along the main scanning direction, that is, the dot distance, varies to cause misregistration or uneven density among superposed color images. This causes color misregistration or color change, thus degrading image quality.
Conventionally, in optical scanners using polygon mirrors, the misregistration as described above is periodically detected at the time of turning on an apparatus or an interval between jobs using detection patterns recorded on a transfer body as disclosed in Japanese Examined Patent Application Publication No. 7-19084 and Japanese Patent No. 3049606. Then, the misregistration is corrected by registering the positions of the first lines by synchronizing writing start timing at every other surface of each polygon mirror. As described above, however, simple replacement of the polygon mirrors, not to mention performing correction, cannot reduce color misregistration or color change.
Meanwhile, Japanese Laid-Open Patent Application No. 2002-258183 discloses a mirror driving method in the case of retaining disposition of multiple vibrating mirrors. According to this driving method, the vibrating mirrors are supported on a single base as a unit with their mirror surfaces being oriented in the same direction, and are driven by setting a common scanning frequency in the band excluding a resonant frequency.
As described above, the existing techniques have a problem in that mere replacement of polygon mirrors cannot reduce color misregistration or color change in the case of applying vibrating mirrors to an optical scanner supporting the “tandem system.”